Project Summary The neurotransmitter oxytocin (OXT) is well known for its social and reproductive roles, but has also gained increasing attention as an endogenous regulator of the neural response to pain. Exposure to noxious stimuli activates both neuroendocrine and centrally-projecting OXT-producing neurons in the mammalian hypothalamus, and both populations have been shown to exert analgesic effects. Additionally, the centrally- projecting group influences pain-related behaviors such as fear conditioning through targets in the piriform cortex and the amygdala. While the ability of these cells to dampen the acute perception and long-term memory of painful experience makes them highly relevant to the clinical treatment of pain disorders, post-traumatic stress, and other conditions, very little is known about the cellular and circuit mechanisms by which hypothalamic OXT neurons influence pain processing, or more generally how they might affect other pain- related phenomena. This project will exploit the experimental leverage offered by the larval zebrafish to investigate the means by which OXT neurons enhance the sensitivity of a sensorimotor circuit to painful stimuli and promote defensive behaviors. Our central model is that stimulus intensity is encoded by graded activity in a subpopulation of OXT neurons which project onto and activate or sensitize spinal projection neurons (SPNs) through the differential release of OXT and glutamate. We will test this hypothesis by using a combination of calcium imaging, optogenetics, electrophysiology, and behavior to: (1) Determine whether the subpopulation of OXT neurons activated by pain includes cells that project onto the SPNs, and whether those neurons specifically mediate OXT?s effects on defensive behavior; (2) Quantify the individual contributions of OXT and co-transmitted glutamate to SPN activation and behavioral sensitization during noxious experience; (3) Determine whether differential coupling of glutamate and OXT release to spike frequency enables the OXT neurons to switch from a simple, excitatory mode of activity to a stronger, modulatory mode at high stimulus intensities. These experiments will show how the basic cellular and circuit properties of oxytocinergic neurons shape the behavioral response to pain in a vertebrate model.